1. Field of the Invention
This invention relates to a method of making a tunable semiconductor laser capable of controlling the wavelength of the coherent light to be output.
2. Description of the Prior Art
The tunable semiconductor lasers is used as a local oscillation light source for receiving in coherent light communications systems, and also as a carrier-wave light source of frequency-division multiplex light transmitter. Not only for increasing channel capacity in optical communications systems but also for enabling longer distance between communication relay stations by longer transmittable distance, it is necessary to improve the performance of the semiconductor laser which is a major component. Effect is being devoted to the development for this.
Tunable lasers proposed so far are of different types such as distributed feedback (DFB) type, distributed Bragg Reflector (DBR) type, and tunable twin guide (TTG) type. DFB type and DBR type of these conventional tunable semiconductor lasers are described, for example, in U.S. Pat. No. 4,949,350 issued on Aug. 14, 1990, and hence the detail of these is omitted herein.
A typical example of TTG tunable semiconductor laser is described in U.S. Pat. No. 5,480,049 issued on Sep. 10, 1991 which is made by the following process: On the surface of a p-type indium phosphide (InP) substrate is a layer sequence consisting of a 2 .mu.m-thick buffer layer of the same material, a 0.15 .mu.m-thick diffraction grating layer of p-type indium gallium arsenide phosphide (InGaAsP), an n-type InGaAsP active layer of 0.1 .mu.m thick, a 0.05 .mu.m-thick antimelt-back layer of InGaAsP, a n-type InP central layer of 0.1 .mu.m thick, a n-type InGaAsP tuning layer of 3 .mu.m thick, a p-type InP cover layer of 1.5 .mu.m thick, and a InGaAsP contact layer of 0.2 .mu.m thick. These are grown by the liquid phase epitaxy technique each on the top of the preceding one. Then the layer sequence is etched to leave a stripe-shaped mesa having a predetermined width. The remaining where etched away is filled as lateral n-type InP regions. The periphery of the lateral regions is inactivated by proton implantation or p-type impurity diffusion. Then each of conducting films insulated from each other is applied by the usual patterning onto the surfaces of the p-type InP substrate, the contact layer, and the lateral regions, respectively, and connected to a laser oscillation drive power input electrode, a tuning power input electrode, and a common lateral layer electrode, respectively. Electric current injection of charge carriers (holes and electrons) bringing about laser oscillation reaches the common electrode from the laser oscillation drive power input electrode via the conductor layer on the substrate undersurface, substrate, barrier layer, diffraction grating layer, active layer, and lateral regions. On the other hand, the current injection for tuning reaches the common electrode from the tunable power input electrode via the cover, tuning, and lateral regions.
A TTG tunable semiconductor laser achieving independent definition of the current path and optical waveguide, respectively, by different means is described in U.S. Pat. No. 5,008,893 issued on Apr. 16, 1991, and its manufacturing process is summarized as follows: On the surface of a p-type InP substrate of 80 .mu.m thick, a multi-layer structure consisting of a 5 .mu.m thick buffer layer of the same material, a 0.15 .mu.m-thick barrier layer composed of three layers of n-type InGaAsP, n-type InP and p-type InP are formed sequencially. Then a part of the multilayer structure is etched away in stripe form corresponding to the current path for supply of carriers for laser oscillation. A 1 .mu.m-thick compensating layer of p-type InP is formed over the overall surface in addition to filling the etched part with the same material, and thereon an active layer of InGaAsP is formed. After applying a 0.03 .mu.m-thick protective film of InGaAsP onto this active layer, an n-type InP central layer of 0.15 .mu.m thick, and an n-type InGaAsP tuning layer of 0.2 .mu.m thick are formed. After forming the first contact layer of n-type InGaAsP of 0.1 .mu.m thick on the entire surface of the tuning layer, a p-type InP cladding layer of 1.5 .mu.m thick and the second contact layer of p-type InGaAsP of 0.2 .mu.m thick are formed. Then the first etching is carried out to remove a part of the periphery of the multilayer structure including from the second contact layer to the active layer to form an edge parallel to the stripes. Then the second etching is carried out to leave a part of the multilayer structure from the second contact layer to the cladding layer to be a ridge waveguide having the same form as the stripe of the current path and to remove the remaining part. Subsequently to the substrate undersurface, and the first and second contact layers, respectively, are connected each of a laser oscillation drive power input electrode, common electrode, and tunable power input electrode. In the TTG tunable semiconductor laser with this ridge waveguide, current for carrier injection bringing about laser oscillation reaches the common electrode from the laser oscillation drive power input electrode through the compensating layer filling the stripe-shaped window in the barrier layer, the active layer and the central layer. On the other hand, the current for carrier injection for tuning reaches the common electrode from the tuning power input electrode through the cladding, and tuning layers.
As apparent from the above-described structure, the TTG tunable semiconductor laser is tunable through variation of the refractive index which can caused by carrier injection from the wavelength control electrode under the condition of single mode oscillation kept by carrier injection from the laser oscillation drive electrode and in turn. The change in refractive index of the tuning layer which varies laser oscillation wavelength is due to production of plasma in the layer resulting from the carrier injection. The tuning efficiency of the TTG laser is dependent on how much carriers are enclosed into the tuning layer and on what extent the laser oscillation light and the tuning layer interact.
The above-described process of manufacturing TTG laser includes a processing step of building a striped-shaped mesa or a waveguide ridge by selective etching of the multilayer structure as described above, and therefore these geometries vary considerably with the temperature, concentration, and agitation of the etchant used in this step. In other words, progress of etching varies finely with site of the multilayer structure and variable processing time, and this makes it difficult to ensure the geometrical uniformity of the stripe-shaped mesa or waveguide ridge. Further the variation in progress in etching causes geometrical disorder of stripe-shaped mesa and waveguide ridge, which in turn results in scattering of laser light, loss of laser light output, and lowered efficiency of the current to laser light conversion. Besides the scattering of laser light reduces the coupling efficiency between the semiconductor laser light and an optical fiber constituting optical light transmission path.